A comparative study of the effects of a monophasic and a triphasic oral contraceptive containing ethinylestradiol (EE) and levonorgestrel (LNG) on lipid and lipoprotein metabolism was conducted on 45 Singapore women under the WHO Special Programme of Research in Human Reproduction. The women were randomly allocated to one of the two pill groups - a triphasic preparation containing low doses of LNG and EE in various proportions and a monophasic preparation of 150 μg LNG and 30 μg EE, and an additional 18 women, choosing to use IUDs, were recruited as controls. Blood samples were taken at admission, 3 and 12 months thereafter. For both pill groups, total cholesterol decreased with duration of use, while HDL cholesterol decreased slightly at 3 months and therafter increased. LDL cholesterol decreased slightly at 3 months, returning to baseline at 12 months for the monophasic group, while remaining unchanged at 12 months for the triphasic group. The monophasic preparation appeared to lower total, LDL and HDL cholesterols to a greater extent at 3 months than the triphasic preparation. Compared with IUD users, for the monophasic group, both total and HDL cholesterols were significantly lower at 3 months and HDL and LDL cholesterol were lower at 12 months. In addition, ratios of HDL cholesterol/LDL cholesterol and HDL cholesterol/total cholesterol were significantly lower at 12 months. Changes in triglycerides were minimal for both pill groups. However, at 3 months, triglycerides for the triphasic group were significantly higher than the IUD users, but this observation was not of any clinical importance since comparison of changes with their respective pretreatment values were not statistically significant. These results indicate that these changes are related to the dosage and estrogen-progestogen ratio of the preparation. Furthermore, the monophasic preparation may have a disadvantage over the triphasic preparation since it appears to cause more disturbance in lipid metabolism. However, it is noted that these changes in lipids and lipoproteins were minimal and did not appear to be of any clinical significance.